R1:d3546f94c15f

Improve diffusion readme layout

Summary: Uses more standard objects and more padding for reading. Removes the ToC, which is visually broken anyways.

Test Plan: Review a README.md in a local repository.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17752